[OS(NH3)4(ETA(2),ETA(1)-(CH2=CHCO2H)2)]2- PREPARATION, CHARACTERIZATION, DETERMINATION OF ACID-DISSOCIATION CONSTANTS, AND KINETICS AND MECHANISM OF ACID-CATALYZED AQUATION( )
Fs. Nunes et H. Taube, [OS(NH3)4(ETA(2),ETA(1)-(CH2=CHCO2H)2)]2- PREPARATION, CHARACTERIZATION, DETERMINATION OF ACID-DISSOCIATION CONSTANTS, AND KINETICS AND MECHANISM OF ACID-CATALYZED AQUATION( ), Inorganic chemistry, 33(14), 1994, pp. 3116-3120
Herein we report on the preparation and characterization and on some a
spects of the chemistry of [Os(NH3)4-(eta2,eta1-(CH2=CHCO2H)2]2+ (1) a
nd [Os(NH3)4(eta2-CH2=CHCO2H)(H2O)]2+ (2) in aqueous solution. The pK(
a) values, as determined by titration, for 1 are 3.20, 6.78, and 9.8 a
nd for the corresponding Os(III) form are 1.76, 3.46, and 7.40. In eac
h case they refer to deprotonation of eta2-acrylic acid, eta2-acrylic
acid, and an ammonia, respectively. For 2, the values are 2.75, 5.27,
and 8.57, and for the corresponding Os(III) form the values are 1.42,
3.65, and almost-equal-to 8. Here they refer to deprotonation of the c
oordinated H2O, Of the eta2-acrylic acid, and of an ammonia, respectiv
ely. Particularly for Os(II) they illustrate the powerful electron-wit
hdrawing effect of the alkene link. The 3+/2+ redox potentials of 1 an
d 2 in acid are 0.59 and 0.69 V vs (SHE), respectively, and they chang
e with pH in accord with the value of pK(a). The aquation of 1 to form
2 is accelerated by HPF6, but the effect reaches a limit. This behavi
or together with other observations lead us to the conclusion that ope
ning of the eta2-bond precedes protonation and that protonation then t
akes place at the pid electrons, thereby labilizing for substitution.
With HCl and HO3SCF3, complications occur which we ascribe to addition
of the anions to the beta carbon after the eta2-linkage opens. In the
se cases protonation of the pid electrons leads to the formation of mo
nohydrides of Os(IV)(paramagnetic), without loss of the eta1-ligand.